overlay: OBS control window via WebSocket API (#773)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds an overlay window widget for controlling OBS over WebSocket.

Add new lines to the FPS widget that shows recording / streaming timers.

Show notifications for major state changes (stream started/stopped,
recording started/paused/resumed/stopped)

## Testing

src/spice2x/overlay/windows/fps.h

@@ -9,6 +9,9 @@ namespace overlay::windows {
     private:
         std::chrono::system_clock::time_point start_time;
 
+        // anchored top-left position for a window of the given size, per FPS_LOCATION
+        ImVec2 anchored_pos(const ImVec2 &size) const;
+
     public:
 
         FPS(SpiceOverlay *overlay);